There are many different Jobs in Final Fantasy XIV. Once you've mastered your main role as a class of your choosing, you can then choose a Job which relates more specifically to your role in groups and solo play and ultimately it is what defines how you like to play. One of the greatest aspects of FFXIV: ARR is the flexibility. Players are not locked into a single job choice, so when reading over the various ones in the game, keep in mind that it is not a permanent choice. Once you've completed the class level requirements and completed the associated quest for the job you are going for, you will be granted a Job Soul Crystal which you will equip onto your character to grant the abilities of that job. This will not lower your level back down to level 1 either! You will retain the level and skills of the class you were set to.